Method of case hardening steel



Patented June 5, 1934 UNITED STATES reunion or case HARDENING s'rmn.Vincent "r. Malcolm, Springfield, Mass., assignor to Chapman ValveManufacturing Company, Indian Orchard, Mass, a corporation ofMassachusetts No Drawing. Application March 3, 1932,

Serial No. 596,567

2 Claims. (Cl. 148-15) This invention r'elates to improvements in steelmanufacture and is directed more particularly to improvements in methodsof providing steel with hardened surfaces and the products thereof. 5The principal objects of the invention are directed to the provision ofimprovements in aethods for treating steels to provide hardenedsurfaces, therefore and according to special fea- .tures of theinvention, steel may be treated in the novel way to bring about thehardening effect at low cost.

The ever increasing demands for better and less costly materials of allkinds in order to produce mechanisms to meet the requirements forstrong- 2 and higher unit pressures have created problems overcome bythe method hereinafter described.

With prior art processes certain specific steels must be treated incertain ways wherefore the results are limited. In carburizing as wellas in nitriding processes the time, element is an importantconsideration because the cost is dependent more or less on the timeconsumed in treatment. According to the novel features of this inventionit is possible to treat a wide range of materials, at least, itis notnecessary to confine the treatment to a particular steel or to make itnecessary to provide a special steel which will be particularly adaptedfor the novel treatment. Not only by means of the invention is itpossible to treat. various steels in a relatively short time as comparedwith nitriding and carburlzing operations but the treatment provides anon-brittle hardened surface of desirable depth which is tough while atthe same time the core of the material is tough and of desirable tensilestrength.

According to this invention, steels of various compositions may betreated. For instance ordinary cold rolled and cold drawn steels,classS. A. E. 1010, 1020, 1030, 1040, with or without high manganese, nickelsteels of any particular composition up to and including 5 nickel,chromiumnickel steels of practically any composition, chrome-tungsten,chrome-molybdenum, chromevanadium and others may be treated by the novelprocess. Steels of the types above mentioned may be fabricated or formedinto objects of various sizes and shapes, such as castings, forgings,tubes, rods, bars, sheets, etc., and. the objects may either be finishedground or they may be hardened previous to the finish grind.

According to the method of this invention, the steel to be treated isimmersed in a bath of nitrogen containing salts which is heated to atemperature ranging from 1400 to 1750 degrees F. or

thereabouts. Steel may be treated withpure er, faster, lighter and saferequipment have resulted in great efforts to meet the needs. These in themetallurgical art which have been largelyeutectic mixtures of varioussalts containing to the maximum temperature of-1750 degrees mentioned.

Sodium cyanide; potassium cyanide. ammonium carbonate and ammoniumnitrate have been found to be satisfactoryfor the purpose of theinvention, but no single salts such as cyanides, particularly sodium andpotassium, are suitably adapted for the novel method. It is thereforepreferable to use a combination of salts rather than asingle salt.

Certain mixtures of salts containing nitrogen are heated within therange stated and during the process of treatment ammonia gas is passedthrough the bath. The decomposition of the ammonia in the bathisbelieved to impart to the steel nitrogen in the nascent form. Thenitrogen penetrates into the steel thus readily forming a solid solutionof iron-nitrogen. The hydrogen from the decomposition of the ammoniabreakup passes upwardly to the top of the bathwhere it burns with abrilliant flame. The process should be carefully regulated on account ofthe possibility of the hydrogen and air forming an explosive compound,and requires complete knowledge of the subject.

Nitrogen has heretofore been considered as having a harmful effect uponsteel in that it lowers in particular the elongation value and re-"sistance to shock. In the method of this invention however experimentsshow that the impact value of the steel is not lowered to anyappreciable. extent, if at all.

No measurements of vaporpressure of these salts have been made but-ithas been definitely established that the operation can be carried outwith no undesirable results without a hood over the bath but for generalapplication a hood may be desirable.

The time for treatment in the specified bath indicates that the time ofimmersion of the steel to develop the proper case depends largely on thesize of the work, the temperature of operation and the depth of the caserequired. It has been discovered when steel is immersed in the bath andheated within the temperature range mentioned,:that it will have a casedepth and an extremely hard surface fully as deep 11* not deeper, thanthat obtained by the so-called nitriding process where a specialnitriding steel is used. At the same time, the hardened surface isdecidedly less brittle than that obtained by the nitriding or anysimilar processes.

It has been demonstrated that the surface hardness ranges from 600 to1000 Brinnell hardness as determined by the well-known Monotromapparatus, varying somewhat on the particular treatment.

In the method of this invention, carbon does not enter into thehardening of the surface layer as it does in the ordinary salt bathtreatment and the action is quite unlike case carburizing which is theprocess of adding carbon to the surface and then hardening by heatingthe work above the critical point and quenching. In the carburizingoperation, parts are usually packed in a granular compound which ismostly carbon and heated to a temperature between 1500 and 1800 degreesfor hours. The carbon is taken up or absorbed by the surface of thesteel so that when parts are later quenched the surface behaves like ahigh carbon steel that has been hardened.

It has been known for many years that heating steel in cyanide has beenused extensively under the heading of cyanide dip, cyanide wash andcyanide reheat. In the first two operations work is preheated in thefurnace and then treated in the cyanide bath from 15 seconds to fourminutes before quenching. In the cyanide reheat, cold work is placed inthe cyanide bath and treated from 10 to 40 minutes. 'These treatmentsprovide a mixed nitrogen-carbon case which more or less increases thewear of the piece, but the case obtained is very light and can only beused where the piece has been thoroughly hardened before casing.

According to this invention a case has been produced having a depthaveraging .020 while a .030 maximum has been obtained by the action ofdecomposition of the nitrogen compound. It is believed that the hardnessis considerably greater than a nitrided case.

As an example an object containing from .15 to .25% carbon and from .1to 1.5% manganese was immersed in a salt bath containing 15 to 40%calcium-cyanide, 20% to 40% sodium nitrate, 10 to 15% barium-carbonateand 5 to 10% sodium-chloride and subjected to a temperature some higherthan the melting point of the combination. In approximately an hourstime a case 7 of extreme hardness having a depth of at least .020 wasproduced. In a similar'demonstration where ammonia gas was used the timewas reduced and it was only necessary to heat the bath to a point abovethe melting point.

The economies involved in connection with the invention have a directbearing on the plant scale operation and the cost is considerably lowerthan by the use of the so-called nitriding or carburizing operations."

Experiments? have demonstrated that steels treated by this process haveextreme hardness, wear-and abrasion resistance as well as chemicalstability, in combination with a tough core having very high impactvalue.

The case produced, as previously explained, is approximately .020 deepand at least more than one-half of this case 'is of extreme hardness andis clearly visible in a fractured specimen wherein the hardnessdecreases gradually until the case merges with the core. The case,obtained by my novel method, is harder than anything previously known incase hardened steels and will readily cut glass. The resistance to wearis very great, amounting to considerably more than carburized andquenched steels. At the same time, the core has toughness, strength andimpact resistance, all of which is desirable.

The hardness of the surface may bemeasured in different ways. It may betried qualitatively with a-file or by means of the Monotron or Vickerstesting machines. pressions show the surface of the specimens to besatisfactorily ductile, but the Rockwell point may puncture the case andshould not be used to measure this particular result. Using the twomachines previously described is preferred.

The process. of the invention gives surface hardness with very little ifany material distortion and the physical properties of the core of thematerial is not injurious lyaected or materially changed.

The hardness and wear resistance is extremely great and the hardness isnot injured by tempering, within reasonable limits while the material isrust-resistant to a desirable extent. The process may be adapted forvarious requirements and may be readily accomplished by selecting theproper steel of the proper composition for the purpose for which it isto be used. According to the invention it is possible to quench from thebath in either oil, brine, water or air and include in this various saltcombinations as may be required for quenching media.

Examination of Rockwell im- By means of the novel method described it istreat steel and provide a case which is equal in hardness, toughness,depth and ductility equal to that produced by prior art methods in amuch less time and the process is not limited to any particular steelwherefore economy in manufacture results.

Various changes and modifications may be made in the practice of theinvention without departing from the spirit and scope thereof.

What I desire to secure and claim by Letters Patent of the United Statesis;

1. The process of treating steel to form ironnitrogen and provide ahardened case which consists in, immersing the steel in a fused saltbath, said bath including from 15 to 40% of calcium cyanide, from 20 to40% of sodium nitrate, from 10 to 15% of barium carbonate and from 5 to10% of sodium chloride, heating said bath to within a temperature rangeof from 1400 to 1750 F., and leading ammonia gas into the bottom of saidbath whereby said gas dissociates to supply nitrogen to the bottom of.said bath and hydrogen to the top thereof.

2. The process of treating steel to form ironnitrogen and provide ahardened case which consists in, immersing the steel in a fused saltbath, said bath including from 15 to 40% of calcium cyanide, from 20 to40% of sodium nitrate, from 10 to 15% of barium carbonate and from 5 to10% of sodium chloride, heating said bath to within a temperature rangeof from 1400 to 1750 F., and leading ammonia gas into said bath wherebysaid gas dissociates to supply substantially free nitrogen and hydrogento said bath.

VINCENT T. MALCOLM.

